Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Dertōna, ae, f., a city in Liguria, now Tortona, Plin. 3, 5, 7, § 49; Brut. ap. Cic. Fam. 11, 10, 5; Vell. 1, 15.